1. Field of the Invention
The invention relates to probe-based measuring instruments, such as scanning probe microscopes (SPMs), that measure sample surface characteristics by monitoring interaction between a tip-bearing probe of the instrument and the sample surface. More specifically, the invention relates to a method and apparatus for improving reproducibility of data acquisition by operating the instrument to clean a contaminated tip.
2. Discussion of the Related Art
Probe-based instruments are well-known for obtaining data relating to the surface characteristics of samples. These instruments include stylus profilometers and scanning probe microscopes (SPMs) including atomic force microscopes (AFMs) and magnetic force microscopes (MFMs). The probes of many of these instruments include a pointed tip that interacts with the sample to provide an indication of the measured surface sample characteristic(s). For example, in the case of an AFM, the probe comprises a cantilever and a tip. One end of the cantilever is mounted on a support such as an XYZ actuator assembly, and the tip is mounted on the opposite, free end of the cantilever. The tip, which usually is generally pyramidal in shape, has a base fixed to the free end of the cantilever and a pointed end portion located opposite the base. In use, the pointed end portion of the tip is brought very near to or into contact with the surface to be examined so as to interact with the sample, and the effects of tip-sample interaction on probe motion are monitored and used to provide a feedback signal that serves both to permit adjustment of probe operation and to provide an indication of the measured sample surface characteristic(s). AFMs of this type are disclosed, for example, in U.S. Pat. Nos. 5,226,801; 5,412,980; 5,519,212; and 5,418,363, all of which issued in the name of Elings et al. and the disclosures of all of which are hereby incorporated by reference by way of background material.
Precise AFM measurements require the use of a sharp tip, i.e., one having a minimal pointed end portion diameter. Dull tips may not be able to probe narrow, deep features in a sample. Moreover, sharp sample features may appear broadened in AFM images produced with dull tips. These problems are exacerbated by the fact that even relatively sharp tips become dull as they wear during repeated measurement cycles. Tip manufacturers therefore usually seek to balance precision with reproducibility. The ideal tip is as sharp as is necessary to obtain the required resolution for the desired application but is not so sharp that it wears too rapidly. Hence, considerable effort has been expended in the design and manufacture of tips having a relatively narrow range of effective pointed end portion radii.
It has been observed that, during scanning, an AFM tip can become contaminated by collecting foreign matter along its length and particularly on its pointed end portion. Tip contamination is thought to come from a variety of sources including the ambient atmosphere, the operator during handling, debris on the surface of the sample, and from the formation of oxides during measurement. Tip contamination may also result from the production of particles during scanning due to scratching or wear of the sample or the tip. Sample-based contamination is becoming increasing problematic because there is a growing trend to make tips from a relatively hard, wear-resistant material such as diamond or a diamond-like-carbon substance in order to extend tip life. When these hard tips are used on a relatively soft sample, such as gold or aluminum, the sample tends to wear more than it would if it were to interact with a soft tip, resulting in the production of relatively high quantities of sample-based contaminants. Moreover, the relatively long life of hard tips provides them with a greater opportunity to accumulate contaminants than is experienced by soft tips that must be replaced more frequently.
Tip contamination can severely degrade the reproducibility of the acquired data. Most contaminants increase the effective radius of the pointed end portion of the tip, resulting in an expansion of the area of interaction between the tip and the sample and a corresponding reduction in the resolution and accuracy of the measurement in the same manner that a dull tip reduces accuracy and measurement. Conversely, contaminants can sometimes extend below the pointed end portion of the tip so as to provide a reduced effective tip end portion radius. The resultant narrowed area of interaction between the tip and the sample can provide a heightened resolution that remains for as long as the contaminants remain below the pointed end portion of the tip without being worn away or wiped off. In either event, measurement resolution and accuracy can vary dramatically from scan to scan and even within scans depending upon the amount and location of contaminants on the tip.
Tips have been cleaned manually in the past by manually immersing the tip in a solvent or by rubbing the tip with a substance, such as a fabric, hair, or wood, that is softer than the tip. Cleaning by hand is inconvenient at best, is of questionable effectiveness, and risks tip damage or destruction if the tip is rubbed too aggressively. Cleaning by hand is also impractical in many instruments, particularly if one wishes to clean the tip without removing the probe from the instrument.
A need therefore has arisen to incorporate measures into an AFM or other probe-based measuring instrument to permit the tip of the probe to be cleaned, preferably automatically, thereby increasing the reproducibility of the instrument""s measurements over a longer period of time.
It is therefore a first principal object of the invention to provide a method of cleaning a tip of a probe-based instrument such as an AFM by operating the instrument in a tip cleaning mode.
Another object of the invention is to provide a method that meets the first principal object and that can be performed automatically, preferably intermittently or as required during data acquisition.
In accordance with a first aspect of the invention, these objects are achieved by providing a method of operating a probe-based instrument in a tip cleaning mode that includes acquiring data in the usual manner and, during a scan operation or after its completion, operating the instrument to cause the probe to interact with a cleaning medium so as to remove contaminants from at least the pointed end portion of the tip. If the material of the sample being measured is softer than the material of the tip, the cleaning medium can constitute the sample itself, and the tip can be cleaned simply by operating the instrument to cause the tip to interact with the sample so as to remove contaminants from the tip or at least forcing them to a level above the pointed end portion of the tip that presents the tip""s effective radius.
If the sample is not considerably softer than the tip, or if other considerations prevent tip cleaning using only the sample itself, the cleaning medium can take the form of a separate object located adjacent the sample. In this case, the instrument can be controlled to move the probe from the test region over the sample to a cleaning region over the separate cleaning medium and to effect the necessary cleaning interaction between the tip and the cleaning medium.
Whether the cleaning medium is the sample or another medium, the cleaning interaction between the tip and the cleaning medium, for example, may take the form of a ramming action, a wiping action, a combination of both, or any other action resulting in contaminant removal.
In accordance with another aspect of the invention, the instrument includes an AFM, in which case the probe includes a cantilever having a fixed end and a free end to which the tip is attached.
Another object of the invention is to provide a probe-based measuring instrument configured to clean the tip of its probe as required.
Other objects, features and advantages of the invention will become apparent to those skilled in the art from the following detailed description and accompanying drawings. It should be understood, however, that the detailed description and specific examples, while indicating preferred embodiments of the present invention, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.